Universality Violation Research Articles

Rare decay t→cγγ via scalar leptoquark doublets

A calculation of the one-loop contribution to the rare three-body flavor changing neutral current top quark decay $t\to c\gamma\gamma$ is presented in the framework of models with one or more scalar leptoquark $SU(2)$ doublets with hypercharge $7/6$. Analytical expressions for the invariant amplitude of the generic decay $f_i\to f_j\gamma\gamma$, with $f_{i,j}$ a lepton or quark, are presented in terms of Passarino-Veltman integral coefficients, from which the amplitudes for the processes $t\to c\gamma\gamma$ and $\ell_i\to \ell_j\gamma\gamma$ follow easily. An analysis of the current constraints on the parameter space is presented in the scenario with only one scalar LQ doublet and bounds on the LQ couplings are obtained from the muon $g-2$ anomaly, the lepton flavor violating (LFV) decay $\tau\to \mu\gamma$ and extra constraints meant to avoid tension between theory predictions and experimental data. For a LQ with a mass in the range of $1$--$1.5$ TeVs, the estimate ${\rm Br}(t\to c\gamma\gamma)\sim 10^{-11}$--$10^{-12}$ is obtained for the largest allowed values of the LQ coupling constants, which means that this decay would be below the reach of future experimental measurements. We also consider an scenario with three scalar doublets, which was recently proposed to explain the lepton flavor universality violation anomalies in $B$ decays as well as the muon $g-2$ anomaly. Although this scenario allows large LQ couplings to the tau lepton and the $c$ and $t$ quarks, the branching ratio of the $t\to c\gamma\gamma$ decay is also of the order of $10^{-11}$--$10^{-12}$ for LQ masses of 1.7 TeV.

The role of long distance contribution to the B → K⁎ℓ+ℓ− in the Standard Model

We investigate rare semileptonic B→K⁎ℓ+ℓ− by looking at a specific long distance contribution. Our analysis is limited to the very small values of physical accessible range of invariant mass of the leptonic couple q2. We show that the light quarks loop has to be accounted for, along with the charming penguin contribution, in order to accurately compute the q2-spectrum in the Standard Model. Such a long distance contribution may also play a role in the analysis of the lepton flavor universality violation in this process.

Constraints on lepton universality violation from rare B decays

The LHCb Collaboration has recently released a new study of ${B}^{+}\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ and $B\ensuremath{\rightarrow}{K}^{*0}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ ($\ensuremath{\ell}=e$, $\ensuremath{\mu}$) decays, testing lepton universality with unprecedented accuracy using the whole Run 1 and 2 dataset. In addition, the CMS Collaboration has reported an improved analysis of the branching ratios ${B}_{(d,s)}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$. While these measurements offer, per se, a powerful probe of new physics, global analyses of $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ transitions also rely on the assumptions about nonperturbative contributions to the decay matrix elements. In this work, we perform a global Bayesian analysis of new physics in (semi)leptonic rare $B$ decays, paying attention to the role of charming penguins which are difficult to evaluate from first principles. We find data to be consistent with the Standard Model once rescattering from intermediate hadronic states is included. Consequently, we derive stringent bounds on lepton universality violation in $|\mathrm{\ensuremath{\Delta}}B|=|\mathrm{\ensuremath{\Delta}}S|=1$ (semi)leptonic processes.

A Light Shed on Lepton Flavor Universality in B Decays

Behind succeeding measurements of anomalies in semileptonic decays at LHCb and several collider experiments hinting at the possible violation of lepton flavor universality, we undertake a concise review of theoretical foundations of the tree- and loop-level b-hadron decays, b→clνl and b→sl+l− along with experimental environments. We revisit the world averages for RD(D*), RK(K*), RJ/ψ, and Rηc, for the semileptonic transitions and provide results within the framework of the relativistic independent quark model in addition to the results from model-independent studies. If the ongoing evaluation of the data of LHC Run 2 confirms the measurements of Run 1, then the statistical significance of the effect in each decay channel is likely to reach 5 σ. A confirmation of these measurements would soon turn out to be the first remarkable observation of physics beyond the Standard Model, providing a wider outlook on the understanding of new physics.

Euclid preparation

Euclid’s photometric galaxy cluster survey has the potential to be a very competitive cosmological probe. The main cosmological probe with observations of clusters is their number count, within which the halo mass function (HMF) is a key theoretical quantity. We present a new calibration of the analytic HMF, at the level of accuracy and precision required for the uncertainty in this quantity to be subdominant with respect to other sources of uncertainty in recovering cosmological parameters from Euclid cluster counts. Our model is calibrated against a suite of N-body simulations using a Bayesian approach taking into account systematic errors arising from numerical effects in the simulation. First, we test the convergence of HMF predictions from different N-body codes, by using initial conditions generated with different orders of Lagrangian Perturbation theory, and adopting different simulation box sizes and mass resolution. Then, we quantify the effect of using different halo finder algorithms, and how the resulting differences propagate to the cosmological constraints. In order to trace the violation of universality in the HMF, we also analyse simulations based on initial conditions characterised by scale-free power spectra with different spectral indexes, assuming both Einstein–de Sitter and standard ΛCDM expansion histories. Based on these results, we construct a fitting function for the HMF that we demonstrate to be sub-percent accurate in reproducing results from 9 different variants of the ΛCDM model including massive neutrinos cosmologies. The calibration systematic uncertainty is largely sub-dominant with respect to the expected precision of future mass–observation relations; with the only notable exception of the effect due to the halo finder, that could lead to biased cosmological inference.

Neutrino masses, flavor anomalies, and muon g−2 from dark loops

The lepton sector of the Standard Model is at present haunted by several intriguing anomalies, including an emerging pattern of deviations in $b \to s \ell \ell$ processes, with hints of lepton flavor universality violation, and a discrepancy in the muon anomalous magnetic moment. More importantly, it cannot explain neutrino oscillation data, which necessarily imply the existence of non-zero neutrino masses and lepton mixings. We propose a model that accommodates all the aforecited anomalies, induces neutrino masses and provides a testable dark matter candidate. This is achieved by introducing a dark sector contributing to the observables of interest at the 1-loop level. Our setup provides a very economical explanation to all these open questions in particle physics and is compatible with the current experimental constraints.

Singlet vector leptoquark model facing recent LHCb and BABAR measurements

Very recently the LHCb experiment released the first measurement of the ratio R(Λc)=BR(Λb→Λcτν¯τ)/BR(Λb→Λcμν¯μ). Moreover, the BABAR experiment reported a new result of the leptonic decay ratio of Upsilon meson ϒ(3S), namely, Rϒ(3S)=BR(ϒ(3S)→τ+τ−)/BR(ϒ(3S)→μ+μ−). Both measurements are below their corresponding Standard Model predictions (deficit), deviating by ∼1.1σ and ∼1.8σ, respectively. In addition, the LHCb recently presented the first search of the lepton flavor violating decay B0→K⁎0μ±τ∓. Motivated by these new data, in this work we study their impact on the phenomenology of the singlet vector leptoquark (U1) model addressing the hints of lepton flavor universality violation in the semileptonic decays of B mesons (B meson anomalies), by carrying out a global fit analysis. In general, we found that a minimal version of the U1 model with a mass of 2 TeV can successfully explain the B meson anomalies, while being compatible with all other flavor observables and LHC bounds. Interestingly, our study shows that the new observables R(Λc) and Rϒ(3S) generate strong tension, leading to non-trivial effects on the global fit. Future improvements at the LHCb and Belle II experiments would help to understand their complementary. Moreover, we also analyze the impact of the expected sensitivity on flavor observables at Belle II to provide a further test of the U1 model. Finally, we study the minimal assumptions under which the U1 model could, in addition, provide a combined explanation of the anomalous magnetic moment of the muon.

Measurement of structure dependent radiative K+ → e+νγ decay at J-PARC

The J-PARC E36 experiment is aiming at searching for lepton universality violation by accurately measuring the ratio of branching ratio of the K + → e + ν(γ) (K e2(γ)) to K + → μ + ν(γ) (K μ2(γ)) decays. The experiment was performed at J-PARC employing a stopped K + method in conjunction with a 12-sector iron-core superconducting toroidal spectrometer. Charged particle momenta were determined by reconstructing charged particle trajectories in the spectrometer. Particle identification for e + and μ + was carried out using an aerogel Cherenkov counter and a lead-glass Cherenkov counter, as well as by measuring the time-of-flight between TOF counters. The peak structure due to K e2(γ) decay was successfully observed in the e + momentum spectrum after the e + requirement in the PID analysis. The structure dependentradiative K + → e + νγ () events were selected by imposing photon hits in the CsI(Tl) calorimeter or the GSC counter. The experimental spectra were accurately reproduced by the Monte Carlo simulation, indicating a correct understanding of the experimental conditions. The Br() value relative to Br(K e2(γ)) was determined from the and K e2(γ) yields corrected for the detector acceptances. The Br()/Br(K e2(γ)) ratio was derived to be 1.19 ± 0.07 stat ± 0.04 syst in the CsI(Tl) analysis and 1.25 ± 0.14 stat ± 0.04 syst in the GSC analysis.

Charming penguins and lepton universality violation in varvec{b rightarrow s ell ^+ ell ^-} decays

The LHCb experiment has recently presented new results on Lepton Universality Violation (LUV) in B rightarrow K^{(*)} ell ^+ ell ^- decays involving K_S in the final state, which strengthen the recent evidence of LUV obtained in B^+ rightarrow K^{+} ell ^+ ell ^- decays and the previous measurements of B rightarrow K^{*0} ell ^+ ell ^-. While LUV observables in the Standard Model are theoretically clean, their predictions in New Physics scenarios are sensitive to the details of the hadronic dynamics, and in particular of the charming penguin contribution. In this work, we show how a conservative treatment of hadronic uncertainties is crucial not only to assess the significance of deviations from the Standard Model but also to obtain a conservative picture of the New Physics responsible for LUV. Adopting a very general parameterization of charming penguins, we find that: (i) current data hint at a sizable q^2 and helicity dependence of charm loop amplitudes; (ii) conservative NP solutions to B anomalies favour a left-handed or an axial lepton coupling rather than a vector one.

Preparations for a search of the muon EDM at PSI

Electric dipole moments (EDMs) of fundamental particles violate time invariance and the combined symmetry of charge and parity (CP). The existence of a large muon EDM (muEDM) is made plausible by tensions with Standard Model predictions for semileptonic decays of heavy meson measured at LHCb, BaBar, and Belle, as well as the muon’s anomalous magnetic moment (AMM). A discovery of the muEDM would manifest CP and lepton flavor universality (LFU) violation, revealing physics beyond the SM (BSM). The most sensitive muEDM search to date provides an upper limit of 1.8 × 10−19 e cm (CL 95%), extracted from high-precision data collected to measure the muon AMM. At the Paul Scherrer Institute, we are setting up a dedicated search for the muEDM using, for the first time, the frozen-spin technique to target an ultimate sensitivity better than 6 × 10−23 e cm. This novel technique increases the sensitivity to EDM-induced spin precession by cancelling the AMM-induced precession with the application of a precisely tuned electric field perpendicular to the muon momentum and the magnetic field. In this configuration, the dominant source of precession is the EDM coupling to the large relativistic electric field in the muon rest frame, generated by its motion in a strong 3 T uniform magnetic field. In a precursor experiment, we will apply the frozen-spin technique in a compact solenoid demonstrating a sensitivity of better than 3 × 10−21 e cm, probing uncharted and otherwise inaccessible territory in BSM theories.

NP effects in Λb → Λc(*) semileptonic decays

In the context of lepton flavor universality violation (LFUV) studies, we study different observables related to the b → cτν¯τ semileptonic decays. These observables are expected to help in distinguishing between different New Physics (NP) scenarios. Since the τ lepton is very short-lived, we consider three subsequent τ-decay modes, two hadronic πντ and ρντ and one leptonic µν¯µντ. This approach enables the differential decay width to be written in terms of visible (experimentally accessible) variables of the massive particle created in the τ decay. We present numerical results for the observables that can be accessed through the visible kinematics for the Λb → Λc and the Λb → Λ˚c p2595q transitions. This work is based on Refs.[1–3].

Formula omitted] from RPV-SUSY sneutrinos

We analyze the lepton flavor universality violation (LFV) in process b→sℓ+ℓ− in an R-parity violating supersymmetric (RPV-SUSY) scenario. The most recent update on RK from the LHCb collaboration suggests approximately 3.1σ deviation from the Standard Model predictions, strengthening the case of LFV New Physics. In this work, we show that RK(⁎) anomaly can be addressed via only sneutrinos within the framework of R-parity violating interactions assuming phenomenologically viable values of the couplings. While our proposed solution is by no means the only solution to these anomalies, it is a phenomenologically plausible one and, notably, quite a minimal one in the context of RPV-SUSY.

Dark sector tensor currents contribution to lepton's anomalous magnetic moment

In this work we consider a model including dark sector bosons interacting through tensor currents with Standard Model leptons. We show that for certain values of the interaction constant this model has the potential of providing an explanation for the discrepancy between theory and experiment, regarding the anomalous magnetic moment of the muon. The effect on the already established measurements for the electron are small and the lepton universality violation is naturally incorporated. Possible experimental searches together with systematic approach to characteristic properties of the final states are discussed.

Probing Lepton Flavor Universality with Pion Decays

Searches for violation of lepton flavor universality are topical due to hints of discrepancies with Standard Model expectations in studies involving muons, B mesons, and beta decays which may be related by tests of the unitarity of the Cabibbo-Kobayashi-Maskawa quark mixing matrix. The status and prospects of pion decay experiments which offer the most precise tests of e − μ universality are discussed.

Searches for dark photons at accelerators

Dark sector models attempt to describe observed dark matter properties, as well as other anomalies such as muon (g − 2), possible violations of lepton universality in B decay, and the possible observation of a new particle by the ATOMKI experiment. There is an extensive program of operating and proposed experiments to search for the new, low-mass, feebly-interacting particles predicted by such theories. I will focus on current searches at accelerators for dark photons, a component of many dark sector models. The results are interpreted in a basic model containing dark photons and dark matter.

Impact of electroweak group representation in models for B and g−2 anomalies from dark loops

We discuss two models that are part of a class providing a common explanation for lepton flavor universality violation in $b\ensuremath{\rightarrow}s{l}^{+}{l}^{\ensuremath{-}}$ decays, the dark matter (DM) problem and the muon ($g\ensuremath{-}2$) anomaly. The $B$ meson decays and the muon ($g\ensuremath{-}2$) anomalies are explained by additional one-loop diagrams with DM candidates. The models have one extra fermion field and two extra scalar fields relative to the Standard Model. The $SU(3)$ quantum numbers are fixed by the interaction with the Standard Model fermions in a new Yukawa Lagrangian that connects the dark and the visible sectors. We compare two models, one where the fermion is a singlet and the scalars are doublets under $SU(2{)}_{L}$ and another one where the fermion is a doublet and the scalars are singlets under $SU(2{)}_{L}$. We conclude that both models can explain all of the previously mentioned issues simultaneously, while satisfying all other flavor and DM constraints. However, there are crucial differences between how the DM constraints affect the two models leading to a noticeable difference in the allowed DM mass range.

Constrained second-order power corrections in HQET: R(D(*)) , |Vcb| , and new physics

We postulate a supplemental power counting within the heavy quark effective theory (HQET) that results in a small, highly constrained set of second-order power corrections, compared to the standard approach. We determine all $\overline{B}\ensuremath{\rightarrow}{D}^{(*)}$ form factors, both within and beyond the standard model to $\mathcal{O}({\ensuremath{\alpha}}_{s}/{m}_{c,b},1/{m}_{c,b}^{2})$, under truncation by this power counting. We show that the second-order power corrections to the zero-recoil normalization of the $\overline{B}\ensuremath{\rightarrow}{D}^{(*)}l\ensuremath{\nu}$ matrix elements ($l=e$, $\ensuremath{\mu}$, $\ensuremath{\tau}$) are fully determined by hadron mass parameters and are in good agreement with lattice QCD (LQCD) predictions. We develop a parametrization of these form factors under the postulated truncation, that achieves excellent fits to the available LQCD predictions and experimental data, and we provide precise updated predictions for the $\overline{B}\ensuremath{\rightarrow}{D}^{(*)}\ensuremath{\tau}\overline{\ensuremath{\nu}}$ decay rates, lepton flavor universality violation ratios $R({D}^{(*)})$, and the Cabibbo-Kobayashi-Maskawa matrix element $|{V}_{cb}|$. We point out some apparent errors in prior literature concerning the $\mathcal{O}(1/{m}_{c}{m}_{b})$ corrections and note a tension between commonly used simplified dispersive bounds and current data.

A global analysis of b rightarrow s ell ell data in heavy and light Z' models

We perform a model-independent global fit to all b rightarrow s ell ell data in the light of recent measurements of the lepton flavour universality violating (LFUV) observables R_{K_S^0}, R_{K^{*+}}, and the updated observables in B_s rightarrow phi mu ^+ mu ^- decay, by the LHCb collaboration. Assuming NP only in the muon sector, we find that the 1D NP scenarios C_9^{textrm{NP}} <0 and C_{9}^{textrm{NP}}=-C_{10}^{textrm{NP}} continue to be the most favoured ones. However, the significance of the favoured scenario C_{9}^{textrm{NP}}=-C'_{9} has reduced and the updated data now marginally prefers C_{10}^{textrm{NP}} > 0 scenario. The 2D scenarios (C_9^{textrm{NP}}, C_{10}^{prime } ), (C_9^{textrm{NP}}, C_{9}^{prime } ) and (C_9^{textrm{NP}}, C_{10}^{textrm{NP}} ), continue to be favoured by the data in the listed order. We analyse generic TeV scale Z' models which can generate the 1D scenarios, C_9^{textrm{NP}} <0 and C_9^{textrm{NP}} = -C_{10}^{textrm{NP}} as well as 2D scenarios (C_9^{textrm{NP}}, C_{9}^{prime } ) and (C_9^{textrm{NP}}, C_{10}^{textrm{NP}} ). We find that all four models provide an equally good fit to the data. We also consider a model with a 25 MeV Z' that couples to muons only and has a q^2 dependent b - s coupling. We study the implications of the current data on the LFUV observables R_{phi }, Q_{4,5} as well as R_{K^{(*)}} in the high q^2. We find that the Q_{4,5} observables have a potential to discriminate between a few favored solutions, and disentangle different heavy and light Z' scenarios.

Prospects for a flavour violating Z^prime explanation of Delta a_{mu ,e}

The apparent tensions emerging from the comparison of experimental data of the anomalous magnetic moments of the muon and electron to the Standard Model predictions (Delta a_{mu ,e}) could be interpreted as a potential signal of New Physics. Models encompassing a light vector boson have been known to offer a satisfactory explanation to Delta a_{mu }, albeit subject to stringent experimental constraints. Here we explore a minimal extension of the Standard Model via a leptophilic vector boson Z^prime , under the hypothesis of strictly flavour-violating couplings of the latter to leptons. The most constraining observables to this ad-hoc construction emerge from lepton flavour universality violation (in Z and tau decays) and from rare charged lepton flavour violating transitions. Once these are accommodated, one can saturate the tensions in Delta a_{mu }, but Delta a_{e} is predicted to be Standard Model-like. We infer prospects for several observables, including leptonic Z decays and several charged lepton flavour violating processes. We also discuss potential signatures of the considered Z^prime at a future muon collider, emphasising the role of the mu ^+mu ^- rightarrow tau ^+tau ^- forward-backward asymmetry as a key probe of the model.

Anatomy of kaon decays and prospects for lepton flavour universality violation

The kaon sector is characterised by several processes which are under active investigation across different experiments. In this work, we present the global picture that emerges from a study of the different decay modes. We begin by revisiting the theoretical component of these decays and providing up-to-date predictions of the Standard Model as well as the corresponding uncertainties. Several new features emerge, in particular for KS,L → mu overline{mu} , and are presented in considerable detail. This offers an ideal platform for extracting the parameter space supported by the existing data. Motivated by possible lepton flavour universality violation in B decays, we investigate such Beyond the Standard Model effects also in rare kaon decays. Without loss of generality, our primary analyses correspond to the paradigm where the Wilson coefficients for operators involving tau leptons are chosen to be equal to that involving the muon, i.e. δCτ = δCμ. We conclude by presenting the possible picture that can be achieved towards the end of the run of data accumulation in the planned experiments. This includes assumptions on possible sensitivity goals that the experiments can aim to achieve, in order to extract the kind of physics highlighted in this paper.